For patients suffering from a variety of injuries or disease states such as cardiac arrest, burns, trauma, heart failure, sepsis, dehydration from any cause, renal failure, or dialysis, it is important to monitor the relationship between the volume of circulating blood and the patient's ability to circulate that volume of blood. Further, in many medical conditions, it is important to know if patients will hemodynamically respond in a favorable manner to providing intravenous fluids and/or if they are volume overloaded. This is especially important in complex states such as sepsis and cardiogenic shock.
However, determination of a patient's intravascular volume status in a noninvasive manner has been problematic. Methods of monitoring cardiac output are commonly used to assess the condition of patient's suffering from a variety of conditions. However, many of the methods that are non-invasive fail to quantify the volume of blood circulating within the patient relative to the patient's ability to circulate that volume. These parameters are important because ideally the physician could adjust the volume of circulating blood (for example via intravenous fluids) in order to achieve optimum cardiovascular circulation or output. Recently, impedance cardiography has been used to measure changes in cardiac output (and thus stroke volume) in response to temporary central fluid provision by raising of the lower extremities. This approach, however, is expensive, generally does not provide sufficient measurement sensitivity or accuracy, and may not be an option to some patients. In particular, impedance cardiography may necessitate that a patient's lower extremities be raised by a health care provider and many further necessitate repetitive raising if used as an endpoint measure. In many instances, raising of the legs will not be possible due to lower extremity injury, pelvic fracture or in situations where the patient may have limb amputation. In addition, passive leg lifting, as a provocative volume challenge maneuver may be ill suited since limb volume will greatly vary between individuals and even potentially within an individual if it is used repetitively when impedance cardiography is used as the end-point of the maneuver. Further, impedance cardiography has not been used to guide a reduction of intravascular volume. Thus, a passive extremity lift when used in conjunction with impedance cardiography as a hemodynamic endpoint cannot be used as a continuous measure to guide therapy.
Another approach, using ultrasound of the inferior and superior vena cava, has been used to look at the changes in these large venous vessels in response to spontaneous and mechanical ventilation with great accuracy. The collapsibility of these large vessels during respiration is indicative of volume status including right atrial pressure and whether or not the patient will increase their cardiac output in response to intravenous fluid administration. However, despite its utility such monitoring is prohibitively cumbersome and expensive and requires an experienced ultrasound operator. Furthermore, ultrasonic measurement of the inferior and superior vena cava cannot be performed continuously for a relatively long period of time. Other technologies like pulse pressure variation and stroke volume variation have been used to examine arterial changes produced by volume induced changes in cardiac output caused by respiration. However, measuring the volume variation of the arterial system has been problematic for various reasons (e.g., various pharmaceuticals may alter arterial vascular stiffness and volume largely independent of total intravascular volume). Additionally, it is unknown whether such a technique will work in patients with very stiff arterial systems from calcific and chronic hypertension conditions. Also such techniques may also require that the tidal volume of the patient be carefully controlled.